Parts Of Object Hidden, Cannot Be Sculpted
This recently happened to me and I believe this is a glitch. When I switch to tab mode I tried to reveal hidden Alt+H  the vertices. When I switch back to sculpting mode, I can't sculpt it.
Again, I believe this is a glitch. I tried to copy the mesh to 2.83 version, unhide the vertices, and copy it back to 2.9 version and it worked.
Edit: after that, whenever I add a mask the it will be invisible. I think it's a bug